Machine readable optical data codes are widely used. Optical data codes are in the form of one-dimensional codes, commonly referred to as "bar codes", and two-dimensional codes, sometimes referred to as "two-dimensional bar codes". Simple, manually scanned readers, such as that disclosed in U.S. Pat. No. 4,603,262, can be provided for one-dimensional codes. More complex readers are needed for two-dimensional codes. These readers are held over the code, while the reader internally scans the code or captures an instantaneous two-dimensional image.
A code can be read as a visible light image or as invisible radiation image. Some optical code readers illuminate visible bar codes with a beam of invisible or "nearly invisible" radiation and detect a resulting fluorescence or reflectance of an indicia. U.S. Pat. No. 4,603,262 and U.S. Pat. No. 4,652,750 teach reading a code by scanning with an invisible beam. U.S. patent application Ser. No. 08/931,575, filed Sep. 16, 1997, discloses the use of a printed invisible encodement on a photographic image to record sound information. The encodement is read by illuminating using a beam of invisible electromagnetic radiation that is subject to modulation by the encodement. The resulting encodement image is captured, decoded, and played back by a data reader.
Commercially available inks are available to be used door the purpose of encoding information upon the surface of a media such as a photographic print. An example of such a commercially available ink is Tennessee Eastman Company part number pm19599/10. This ink exhibits a fairy linear light input versus light output transfer function, so it is understood that illuminating the ink with twice as much light energy results in twice the amount of fluorescent light output from the ink.
Readers commonly include an illumination source. Using an array of light emitting diodes (LED's) to illuminate an area is well known. U.S. Pat. No. 5,319,182 by Havens et. al., discusses the use of an integrated source-image sensor matrix in which an array of photonic devices can be configured to both emit light and detect light, for the purpose of reading indicia. Depending upon the wavelengths of illumination necessary, LED arrays can be very costly. Large amounts of light are generally needed to obtain a satisfactory signal to noise ratio. For example, with a luminescent dye or ink, the illuminated surface must emit enough fluorescent light to sufficiently stimulate the image sensor used to read the fluorescent output emission. It is conceivable that many different inks or dyes can be used to encode data on a medium. A variety of factors such as temperature, humidity, ambient light and time which can cause the output fluorescence of the inks or dyes to change, making encoded data less detectable. To the signal to noise requirement for the dye or ink, must also be added an additional factor for system losses from filters and the like.
Another consideration is minimizing the number of LED's to reduce cost, limit size and maximize efficiency. The ability of an LED to dissipate the electrical power consumed by the production of photons is limited. While packaging and heat management practices such as heat sinking can help, the power consumed, which manifests itself by a buildup of heat in the semiconductor junction, is the culprit which ultimately destroys the LED. A need for high amounts of light energy requires additional LED's for the production of those photons. The additional LED's allow for more illumination capacity, but also increase the power requirements of the system by producing more waste heat. The additional LED's also increase manufacturing cost.
Since it is highly desirable to read encoded media with portable equipment, it is apparent that simply adding more LED's to an array, in an effort to increase illumination, will substantially increase size and cost. Additionally, multiple LED's consume large amounts of power and are best avoided in portable applications.
U.S. Pat. No. 5,717,968 discloses a camera in which an LED, used to write an optical code on film, is powered at a current level beyond its continuous operating limits.
It is, therefore, desirable to provide an improved method of illumination of a fluorescent indicia which enhances illumination capacity while limiting the size of a data reader.